In cellular mobile systems like GSM (but also its successors) the access by a mobile to the network is normally performed via a shared resource medium, called Random Access Channel (RACH). In case the UE initiates a voice call, it first contacts the mobile network via the RACH and typically gets subsequently a Traffic Channel allocated to be used for carrying the voice call. In UMTS the initial phase is similar and the terminal, or user equipment (UE), gets a so-called Dedicated Channel (DCH) assigned to carry the voice call.
In order to prevent UEs from accessing an overloaded cell the concept of Access Classes was invented when GSM was specified [3GPP TS 22.011]. This concept was also applied for UMTS. For this concept each SIM card is assigned to a so-called “Access Class” which is configured by the operator into the SIM card initially. 10 Access Classes are defined for ordinary users [3GPP TS 22.011] and are normally equally distributed to the population of SIM cards. Besides these Access Classes (AC) ranging from 0 to 9, a specific AC 10 is defined for Emergency Calls and 5 specific ACs for operator and other purposes. AC 10 is not assigned to a SIM card, but used by a terminal when placing an emergency call. AC 11 to 15 are assigned additionally to 1 Access Class out of 0 to 9 based on operators policies. Details can be found in [3GPP TS 22.011].
In order to prevent access to a cellular basestation in certain situations (i.e. overload situations) it is possible to bar access to the network by blocking RACH access attempts using Access Class Barring—for UMTS system this principle is described in [3GPP TS 25.331]. A bitmap of 16 bit ranging from 0 to 15 is indicated on the Broadcast Control Channel of the UMTS cell. In case a specific portion (out of 0 to 9) or defined ACs 10 to 15 should be disallowed for access, the appropriate bit is set to “access barred”.
Based on the equal distribution of AC between 0 and 9 it is possible to randomly block access to the cell in 10% steps (10% per individual AC between 0 and 9). As the assignment of ACs to SIM cards is randomly and also the distribution of terminals in given cell is randomly, it is currently with prior art only possible to block a certain percentage of access attempts randomly, without considering the subscription profile or the application requirements. As this was not a problem in former days where mobile networks were mainly used for placing voice calls, it is now required to enable a finer control over the access barring in case of load situation, due to the wide variety of different services and subscriptions in cellular networks.
When Enhanced-UTRAN (also known as “LTE”) was being defined, the concept of access class barring was changed from a discrete assignment of Access Classes to SIM cards and a new concept for determining if access to a cell is applicable has been introduced. Details are defined, e.g., in [3GPP TS.36.331v9.2.0], paragraph 6.3.1, in relation to SystemInformationBlock Type 2. By doing this, the explicit relationship between AC barring and Access Class assignment to a particular UE is removed. Instead of equally distributed AC between 0 . . . 9, an Access Barring Factor (ABF) concept has been defined, where the Access Probability can be defined in finer than 10% steps and the barring only applies in case the UE will really initiate an access toward the network.
As can be seen in the current state of the art above, Access Barring Info is provided in E-UTRAN System Information Block Type 2 with the Information Element (IE) ac-BarringInfo. The struct consists of ac-BarringForEmergency, ac-BarringForMO-Signalling and ac-BarringForMO-Data. The first relates to AC 10 barring for Emergency Calls and can be set to “allowed” or “not allowed” (Boolean). The second applies specifically for establishment of signalling connections and was first introduced with the standardisation of E-UTRAN due to the fact that Access Barring for Mobile Originated Data might be applicable with a high likelihood and thus without separation also access for signalling connection would be prevented, resulting in the fact that a UE would also not be reachable in certain situations. The latter applies for Mobile Originated Data in E-UTRAN—which means any user data transmission originating from the terminal as E-UTRAN only support a Packet Service Domain. Hence it also applies for voice services using a Voice over IP (VoIP) data bearer.
As can be seen from the state of art, besides a separation between Emergency Calls, MO-Signalling and MO-Data, no further distinction for Access Class barring is applicable.
With the advent of smartphones—thus a wide range of applications with totally different traffic characteristics—as well as with the new field of machine-to-machine (M2M) communication, characteristics of access attempts to a cellular network have been completely changed. In contrast to former GSM systems, where placing voice calls by a human being was the main use case of mobile usage, now applications start to access and leave the cellular system much more frequently and thus new means of controlling the access to shared cellular resources—like, e.g., the Random Access Channel—of a cellular system are required. Especially two situations need to be considered by a solution to improve the state of the art:
First, the access to a shared medium does not require human interaction anymore—i.e. if placing a voice call would have failed a couple of times, the user simply would give up, while applications simply continue to try to access if no further preventing means are implemented.
Second, with the growing number of M2M devices, a huge amount of terminal devices in a given cell might request access to the shared resources simultaneously (e.g. to report a status every full hour or when a huge amount of M2M devices moves through the network and performs a location area update).